Not applicable.
Not applicable.
The present invention is directed to production of carbon monoxide (CO). In particular, the present invention is directed to production of high purity CO by integration of CO2 Temperature Swing Adsorption (TSA) and CO Sorption Enhanced Reaction (CO-SER)units.
There is a need in the gas production industry for a more cost-effective process for the production of CO. Current technology is based on steam reforming or partial oxidation of hydrocarbon feed gas to obtain CO. Both technologies utilize high temperature reformers, and produce a relatively impure CO product stream which must be purified by cryogenic distillation or vacuum swing adsorption technology. Both of these approaches are capital and energy intensive.
It would be useful to the industry if an alternative CO production process could be developed which could produce high purity CO product without the need for high temperature reactors and expensive separation equipment.
Carvill et al. (EP 0 737 648 A2) describes a generic Sorption Enhanced Reaction (SER) process concept where the conversion of an equilibrium-controlled gas phase reaction is enhanced and relatively high purity product is produced by cyclically removing a by-product of the reaction by adsorption onto a solid adsorbent. Specifically, an SER process for producing CO from CO2 and H2 via the reverse water gas shift reaction (CO-SER) is described wherein a parallel assembly of reactors containing a physical mixture of water adsorbent and water gas shift catalyst is utilized. The reaction is carried out at a low temperature of 200-400 degrees Celsius. Adsorption of water from the reaction zone shifts the equilibrium-controlled reaction to essentially complete conversion of the feed components, and results in production of relatively high purity CO product gas. Although this CO purity is much higher than could be achieved with a catalyst-only reactor operated at the same conditions, which is, for example, 5.5% at 275 degrees Celsius (see example 2 of Carvill et al.), the SER product could still contain percentage levels of unreacted feed gases, H2 or CO2 (see, e.g., 98 and 97% CO illustrated in Example 1 of Carvill et al.).
The CO-SER process can be used to produce relatively high purity CO (e.g., 98%) from a feed gas consisting of 50% CO2 and 50% H2. The CO is produced by the reverse water gas shift reaction, CO2+H2=CO+H2O. The adsorption of water from the reaction zone drives the local conversion of CO2 and H2 towards completion. The purity of the product gas is typically determined by breakthrough of one of the components of the feed. In an ideal system with no CO2/H2 adsorption on the adsorbent or catalyst, CO2 will break through with the product CO if slightly CO2-rich feed gas is used, and H2 will break through with the product CO if slightly H2-rich feed gas is used. Attainment of higher purity product is possible in principle by terminating the sorption-reaction step before significant breakthrough of CO2 or H2, but this decreases the reactor productivity.
Integration of the CO-SER concept with conventional reforming process technology has been described by Hufton et al. in European Applications EP 0737 647 B1 and EP 0742 172 B1. In both cases, the CO-SER process was added to process a feed gas produced from conventional CO process equipment. In the first case, a CO-SER unit was used to convert CO2 and H2 in effluent gas from a steam methane reformer (SMR) into additional CO. The effluent from the CO-SER unit was then passed to a conventional CO separation unit for product recovery. In the second case, a CO-SER unit was used to produce CO from CO2 and H2 present in the waste gas from a CO-VSA/H2-PSA system. These integrated processes were developed to improve the overall yield of CO from the CH4 fed to the SMR system, and not to improve the purity of the CO-SER product.
The present invention incorporates the operation of an SER process unit for CO production with a CO2 Temperature Swing Adsorption (TSA) unit used for final CO cleanup.
The present invention describes a modification to the operation procedure and flowsheet of the Carvill et al. process wherein a high purity CO product (99.9+%) can be produced by integration of a unique CO purification system with the rest of the CO-SER process. The CO-SER is operated to produce CO (with  less than 5% CO2 as the major impurity) from a feedstock of H2 and CO2. This gas is fed to the CO2 TSA unit to produce high purity (99.9+%) CO.
The processes are integrated through the TSA regeneration step. Regenerant H2 used to heat and cool and purge the TSA beds is used as the feed gas to the CO-SER unit. This unique approach is feasible because (1) the TSA unit can be purged at high pressure (the CO-SER reaction pressure), (2) the TSA process steps are designed to reduce the level of CO in the H2 effluent gas, and (3) CO2 can be tolerated in the H2 feed to the CO-SER unit.
A process for producing high purity carbon monoxide product gas is provided which includes the steps of providing a CO-SER unit, feeding a feed gas of an equimolar mix of CO2 and H2 with a slight excess of CO2 to the CO-SER unit to produce a CO-SER product gas of CO, a small amount of CO2,and substantially no H2 at high pressure, providing a TSA unit having a plurality of adsorber vessels, each adsorber vessel having an adsorbent capable of selectively adsorbing CO2, the adsorber vessel being at high pressure and ambient temperature, and feeding the CO-SER product gas to one of the adsorber vessels in the TSA unit to selectively remove CO2 gas to produce a TSA product gas that is of high purity and of high pressure. The feeding continues to the one adsorber vessel until a point prior to CO2 breakthrough occurring, wherein the adsorbent is substantially spent. The other adsorber vessels are then sequentially fed. The process further includes regenerating any adsorber vessels having adsorbent that is substantially spent and providing H2 for the feed gas in a regeneration process which includes heating the spent adsorbent by passing H2 at high temperature and high pressure to produce a TSA regeneration effluent gas of H2, CO2 and low levels of CO that is of sufficient purity to be used as H2 in the feed gas in a CO-SER process where the TSA effluent gas is provided at high pressure and ambient temperature, and combining the TSA effluent gas with a feed of CO2 to obtain the feed gas of substantially equimolar mix of CO2 and H2 with a slight excess of CO2 and small amount of CO.
Alternatively, the regenerating steps may also provide fuel for a fired heater in the CO-SER. The regenerating steps include heating the adsorbent in at least one of the adsorber vessels having substantially spent adsorbent therein by passing H2 at high temperature and high pressure through the adsorber vessel to eliminate a substantial portion of CO from the adsorber vessel (in the form of void gas and some coadsorbed CO) and producing a first TSA regeneration effluent gas containing H2, a small amount of CO2 and smaller amount of CO at high pressure and ambient temperature; providing the first TSA regeneration effluent gas for fuel for the CO-SER unit; continuing to heat the spent adsorbent by passing H2 at high temperature and high pressure to produce a second, more H2-rich TSA regeneration effluent gas of H2, CO2 and low levels of CO that is of sufficient purity to be used as H2 in the feed gas in a CO-SER process, where the second TSA effluent gas is provided at high pressure and ambient temperature; and combining the second TSA effluent gas with a feed of CO2 to obtain the feed gas of substantially equimolar mix of CO2 and H2 with a slight excess of CO2 and small amount of CO. The adsorbent in the at least one adsorber vessel is then cooled by passing virgin, substantially 100% pure H2 at high pressure and ambient temperature through the adsorber and producing an effluent of H2 gas at a temperature between high and low temperature and high pressure. If neccessary, the temperature of the effluent of H2 gas obtained in the step of cooling said adsorber to high temperature is adjusted to high temperature. The effluent of H2 gas is provided as the H2 in the step of heating the adsorbent in the adsorber vessels having substantially spent adsorbent therein to produce a first TSA regeneration effluent gas and in the step of continuing to heat said spent adsorbent by passing H2 at high temperature and high pressure to produce the second, more pure TSA regeneration effluent gas. A plurality of the steps of feeding the CO-SER product gas to one of the plurality of adsorber vessels and the regenerating steps of heating the adsorbent in at least one of the adsorber vessels, continuing to heat the spent adsorbent, and cooling the adsorbent in the adsorber vessel may occur simultaneously where each step occurs in at least one of the plurality of adsorber vessels.